Waste treatment of metal plating solutions

ABSTRACT

An efficient and economical process for the waste treatment of spent metal plating solutions, primarily spent electroless nickel solutions, to significantly reduce the metal content therein, so as to permit direct discharge to municipal water systems without violation of the law. The waste treatment process is a sequential two-step chemical precipitation process, whereby the dissolved metal content is precipitated first by sodium borohydride reduction, followed by sodium dimethyldithiocarbamate precipitation, with an intermediate filtration step and pH adjustment step interposed there between. The amount of metal-bearing sludge produced is minimal. Furthermore, the bulk of the metals in the sludge may be recovered and reused, resulting in significant reduction in hazardous waste disposal costs.

FIELD OF THE INVENTION

This invention relates to a process for the waste treatment ofmetal-bearing solutions to substantially eliminate the dissolved metalcontent in such solutions prior to discharge to the environment and,more particularly, to a process for the waste treatment of spent metalplating solutions which will enable such solutions to be discharged asnon-toxic waste directly to a municipal wastewater treatment plant.

BACKGROUND OF THE INVENTION

Solutions capable of plating metals onto substrates are widely used inindustry. The most commonly used metal plating solutions includeelectrolytic and electroless solutions.

Electroless nickel plating solutions, in particular, have come intowidespread usage in the manufacture of computer memory discs. Suchsolutions generally contain a nickel metal salt, such as the sulfate,acetate, carbonate or chloride salt, for the source of dissolved metalplating ions, a reducing agent, such as sodium hydrosulfite, sodiumhypophosphite, sodium borohydride, boranes or hyrdazines, to reduce themetal ions to metallic form, a complexing or chelating agent, such asmonocarboxylic acids, dicarboxylic acids, hydroxycarboxylic acids, aminoacids, and alkanolamines, to maintain the metal ions in solution andprevent premature precipitation, and a pH adjuster to maintain thesolution pH typically within an acidic range of 4-5. Accelerators,stabilizers, buffers, and wetting agents may also be included in theelectroless nickel solutions.

These electroless nickel solutions only have a limited useful life andeventually become depleted or spent. Typically, after a limited numberof plating cycles, the concentration is reduced of both the complexedmetal in solution (by plate-out onto the substrate) and the reducingagent (by consumption according to the chemical reaction controllingplating) to where the plating rate of the electroless bath is slowedsufficiently to become unsatisfactory, and the bath has to be discarded.

Disposal of such spent electroless nickel solutions, however, presents amajor problem, since a large percentage of the original complexed nickelcontent remains dissolved in the spent solutions. From an environmentalstandpoint, it is well known that heavy metal ions and particularlythose of the metal here of interest, namely nickel, can have adversetoxic effects on the environment if directly discharged in soluble forminto effluent wastewater streams that feed into municipal water systemsor natural bodies of water. Also, such highly chelated streams generallycannot be mixed with unchelated streams at the wastewater treatmentplant, making processing much more difficult. As a result, dischargingdissolved heavy metals, including nickel, directly into effluentwastewater streams, in other than minute quantities, is prohibited bylocal, state, and federal regulations.

A number of wastewater treatment processes have been proposed to reducethe metal content in spent electroless solutions to low levels prior todischarge. Previously, many users of electroless baths simply dosedtheir wastes with caustic soda to precipitate the bulk of the heavymetal contaminant as insoluble hydrous oxides (metal hydroxides),whereupon the hydrous oxide sludge was pressed into a filter cake,drummed, and disposed. This method of metal removal, however, was knownto produce a very large quantity of metal-bearing sludge, all of whichneeded to be disposed of in approved hazardous landfills. Sludgedisposal is very expensive, environmentally detrimental, wasteful ofnatural resources, and involves compliance with local, state and federalregulations. In addition, the sludge producer assumes perpetualliability for the sludge deposited in the landfill site which isundesirable. Another problem with this treatment method is that it isinefficient and generally did not work well due to the high chelantlevels in the spent solutions.

Another waste treatment method previously used for spent electrolesssolutions was to simply electrolessly plate out the metal by dosing thesolution at slightly alkaline pH with reducing agents. The reducingagents typically used to convert the dissolved metal salt into insolublemetal precipitate include sodium borohydride, sodium hydrosulfite,sodium hypophosphite, boranes and hydrazines. An advantage of thismethod is that the metal-bearing sludge produced contains high levels ofelemental metal which can be reclaimed by smelting and sold at a profit.Therefore, hazardous waste disposal of the sludge is no longer required.Plainly, this method is economical, environmentally friendly, andconserves natural resources. One drawback, however, is that the solublenickel content in the treated electroless nickel baths can only bepartially reduced. Generally, the nickel content can only be lowered toabout 3-70 parts per million (ppm) which no longer meets the dischargelimits in most jurisdictions.

Still another prior waste treatment method known for reducing thedissolved metal content of spent electroless baths to acceptabledischarge levels involves organosulfur precipitation of the metal bydosing the spent solution at a pH of 5-8 with water-soluble sodiumdithiocarbamate (DTC) precipitating agents, such as sodiumdimethyldithiocarbamate (DTC-Na). The dissolved metal salt complexeswith the soluble dithiocarbamate salts to form insoluble metaldithiocarbamate precipitates. The metal dithiocarbamate particles aregenerally very fine and not conducive to settling, and typically requirethe aid of coagulants and flocculants to form larger, faster settlingflocs which are more capable of removal by filtration. Thedithiocarbamate method effectively removes the heavy metal content inspent electroless baths to non-detectable levels, but undesirablyproduces huge quantities of heavy metal-bearing carbamate sludge whichcreate potentially dangerous hydrolytic loading on the typicalliquid-solid system. Furthermore, these sludges must be classified ashazardous waste and disposed of predominantly in approved hazardouslandfills. Here again, sludge disposal is very expensive,environmentally detrimental, and wasteful of natural resources.

Prior attempts to waste treat spent electroless nickel solutions withborohydride reduction followed by dithiocarbamate precipitation withoutremoving the reduction precipitants or lowering the pH prior todithiocarbamate precipitation have generally been met without muchsuccess. With the combined method, no appreciable differences in thenickel levels have been shown over straight borohydride reduction, whichlevels fall above current discharge limits.

While the prior processes reduce the metal content of the spentelectroless plating solutions, the need still exists for efficient andeconomical methods for waste treating such solutions which also meetcurrent discharge limits in the low parts per million withoutsimultaneously generating significant amounts of hazardous waste.Moreover, as the regulatory rules for waste stream effluent dischargelimits become more stringent, such as to certain fractional parts permillion, a search for improved processes to remove the metal contentbecomes mandatory.

SUMMARY OF THE INVENTION

It is an object of this invention, therefore, to provide an efficientand economical process for the waste treatment of metal-bearing wastesolutions, particularly spent electroless nickel plating solutions thatcontain dissolved nickel, sometimes other metals, and complexing agents,to substantially remove the dissolved metal content therein, so as toleave the treated solutions classifiable as non-polluting waste by lawand suitable for discharge directly to a municipal wastewater treatmentplant or a natural body of water.

It is another object of this invention to provide a waste treatmentprocess that is simple in operation and employs an effective,sequential, two-step chemical treatment method involving chemicalreduction, particularly borohydride reduction, followed by organosulfurprecipitation, particularly dithiocarbamate precipitation, and thatsurprisingly meets current discharge limits.

It is a yet another object of this invention to provide a wastetreatment process that generates reduced amounts of hazardous sludge,resulting in a significant reduction in hazardous waste disposal costs.

Yet another object of this invention is to provide a waste treatmentprocess which conserves natural resources by generating, for the mostpart, a sludge containing high levels of solid metal compounds,including elemental metal ore, which can be recovered and reclaimed,rather than disposed of as a hazardous waste.

Still another object of this invention is to provide a waste treatmentprocess which reduces the dissolved metal content down to the low partsper million and, in most cases, down to fractional parts per million.

And still another object of this invention is to provide a wastetreatment process which can treat multiple metal-bearing wastewaterstreams, which previously required separate treatments.

Yet another object of this invention is to provide an apparatus suitablefor use in performing such a waste treatment process.

In accordance with this invention, a process is provided for the wastetreatment of metal-bearing wastewater solutions containing dissolvedchelated or non-chelated metals, particularly electroless nickel platingsolutions, whereby the treated solution can be discharged directly to amunicipal wastewater treatment plant or to the environment. Broadlystated, the process comprises the sequential steps of chemicalreduction, followed by organosulfur precipitation, with an intermediatefiltration step and pH adjustment interposed there between. In apreferred embodiment, the first step comprises borohydride reduction andthe second step comprises dithiocarbamate precipitation. The heavy metalcontent in the precipitated matter produced in the first stage of thisprocess can ultimately be recovered as metal, essentially eliminatingthe need for large volumes of hazardous waste disposal.

The various objects, features and advantages of this invention willbecome more apparent from the following description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With this description of the invention, a detailed description followswith reference made to the accompanying drawing in which:

FIG. 1 represents a flow chart showing the currently preferred processand apparatus for waste treatment of spent electroless nickel solutionsin accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

This invention provides a sequential, two-step, simple, efficient,economical, and environmentally favorable, waste treatment process formetal-bearing wastewater solutions, particularly spent electrolessnickel plating solutions, which can lower the dissolved metal content insuch solutions to acceptable levels, generally on the order of about ca.≦3 ppm and preferably ca. ≦0.5 ppm, so that the solutions can bedischarged directly into a wastewater treatment plant stream or anatural stream.

The metal-bearing solutions to be waste treated may come form a varietyof sources such as metal process streams from mining and metallurgicaloperations, electrolytic and electroless metal plating baths, othermetal recovery operations, such as ion exchange, and chemicalmanufacture, and the like. Metal-bearing solutions containing heavymetals, such as nickel, copper, cobalt, silver, gold, lead, mercury,platinum, and any other metals reduced or precipitated by alkali metalborohydrides may be treated according to this invention. Such metals canbe found throughout the transition metals, lanthanides, actinides, andpost-transition metals. Other metal-bearing solutions that contain heavymetals, such as zinc, which are not reduced by alkali metal borohydridesbut can be precipitated by alkali thiocarbamates may also be treatedaccording to this invention. However, the invention is particularlyeffective in treating metal-bearing waste solutions containing dissolvednickel. Thus, for convenience, the following description of thisinvention will be directed to the treatment of discarded or spentelectroless nickel plating solutions.

Spent electroless nickel solutions to be treated usually containsubstantial quantities of dissolved nickel. In general, the dissolvednickel content can vary within very wide limits dependent upon thesource of the specific solution treated. For purposes of illustrationonly, the nickel content of a conventional spent electroless nickelsolution to be treated in accordance with this invention usually variesbetween about 4,000-6,000 ppm (4-6 grams/liter). Consequently, asuccessful waste treatment process permitting direct disposal of thesolution to the environment should comprise lowering the dissolved metalcontent in the solution to non-toxic discharge levels generally on theorder of ca. ≦3 ppm Ni, and preferably ca. ≦0.5 ppm.

The waste treatment process of this invention is amenable to batchwiseor continuous processing, although batchwise processing is presentlypreferred.

In accordance with this invention, the first step in the waste treatmentof the spent electroless nickel plating solution is removal of thedissolved metal from solution by chemical reduction, particularlyborohydride or borohydride catalyzed reduction. The spent electrolesssolution to be waste treated is first contacted with a reducing agentfor a sufficient time to cause the dissolved metal salt to undergochemical reduction, resulting in the precipitation of metal compoundsout of the solution. A preferred method for accomplishing this is by theaddition of an alkali or alkaline earth metal borohydride, preferablysodium borohydride from an economic standpoint, to the spent electrolessnickel solution. The sodium borohydride is usually added as an aqueoussolution of sodium borohydride and sodium hydroxide (for hydrolyticstability of the borohydride ion and reduction of hydrogen gasevolution). An example of a suitable hydrolytically stabilizedborohydride reduction solution is sold under the trademark VenMet® byMorton International, Inc. of Chicago, Ill.

The amount of sodium borohydride solution used will vary depending uponthe particular system for which treatment is desired and will mainly beinfluenced by the metal content of the waste solution and the presenceof other ligands and chelant molecules.

Although stoichiometric quantities of sodium borohydride vis-a-vis thedissolved metal content may be employed, it is generally preferred toemploy a stoichiometric excess amount of sodium borohydride tocompensate for consumption during competing oxidative side reactions.Normally, the sodium borohydride solution is added in a sufficientamount to provide at least about 4 to 15 times the amount of sodiumborohydride required to reduce the waste metal in the spent electrolessnickel solution. In some cases less than stoichiometric levels ofborohydride can be employed as the borohydride reduced nickel oftencatalyzes additional hypophosphite reduction of the chelated nickel ionsin solution.

Adjustment of the pH of the electroless nickel solution before theaddition of sodium borohydride is generally desired to maximize the rateof sodium borohydride reduction. For systems in which the primary metalion is nickel, the borohydride reduction can be carried out in a pHrange of between about 4 and 11 and preferably in a range of between 7and 11. The pH adjustment is typically accomplished with the addition ofsufficient quantities of alkaline agents, such as sodium hydroxide,calcium hydroxide, magnesium hydroxide, and lime, or mixtures thereof,to the electroless nickel solution before the addition of the sodiumborohydride solution. Normally between about 200 and 2,000 ppm of limeis employed and then final pH adjustment is made to the solution.

It may also be desirable to add sufficient amounts of oxidizerscavenging agents, such as sodium bisulfite, sodium metabisulfite, andsodium sulfite, or mixtures thereof, to the reaction mixture in order tominimize consumption of the sodium borohydride prior to metal reduction.This material is typically added to the spent solution at a low solutionpH of about 4-5 prior to final pH adjustment. Normally between about 200and 2,000 ppm of bisulfite is employed in this first treatment stage.

The treated solution is allowed to settle and the nickel-bearingprecipitate which results from such treatment is separated from thetreated electroless solution by conventional solid-liquid separationtechniques, such as decantation or filtration. It should be understoodthat in order to run the process of this invention in an effectivemanner, the inventors have found that it is critical to separate theprecipitated matter from the treated liquid prior to the subsequentchemical treatment of the liquid. Typically, the precipitate is allowedto settle and is then withdrawn from the bottom of the reaction vesselas a sludge. The sludge is then passed through a filter press toseparate the precipitate as a filter cake from the treated liquid. Thefilter cake which contains high levels of metallic nickel compounds isdesirably transported to a smelting operation to recover elementalnickel which can be reused in various chemical processes, instead ofbeing disposed as hazardous waste in a landfill.

The dissolved nickel content in the essentially precipitate-free treatedsolution following borohydride reduction is typically between about ca.3-70 ppm.

Following chemical reduction, the second step in the sequentialtreatment process of this invention is the removal of substantially allof the remaining dissolved metal content from the treated solution byorganosulfur precipitation, particularly thiocarbamate precipitation. Asmentioned above, the solution prior to this treatment stage isessentially free of precipitated matter generated from the firsttreatment stage. This prevents the reduction precipitate frominterfering with the subsequent thiocarbamate precipitation, and allowsfor direct contact between the precipitating agent and the remainingnickel in the treated solution. Furthermore, cross-contamination ofprecipitates is eliminated, which preserves the high purity of thereduction precipitate.

In the second stage, the treated electroless solution is contacted withan organosulfur precipitating agent, such as a thiocarbamate, for asufficient time to form an insoluble metal organosulfur precipitate. Aconvenient method for accomplishing this is by the addition of awater-soluble alkali and alkaline earth metal polythiocarbamate to thetreated electroless nickel solution. Examples of particularly usefulwater-soluble alkali metal polythiocarbamates include sodiumdimethyldithiocarbamate, sodium diethydithiocarbamate, sodiumtrithiocarbamate, and the like. Generally, the alkali metaldialkyldithiocarbamates are most desirable, particularly sodiumdimethyldithiocarbamate. Specific examples of sodiumdimethyldithiocarbamates are those sold under the trademark Metal Plex™143 by Morton International, Inc. of Chicago, Ill.

Although stoichiometric amounts of the sodium thiocarbamate vis-a-visthe metal content may be employed, it is generally preferred to employ astoichiometric excess of this precipitating agent. Generally, the sodiumthiocarbamate is used in an amount sufficient to provide at least about20 times the amount required to precipitate the waste metal as insolublenickel thiocarbamate complexes.

Adjustment of the solution pH before precipitation is effected isgenerally desired. The pH of the electroless solution is typicallylowered to between about 5-8 prior to the addition of the thiocarbamateprecipitating agent with appropriate pH adjusters, such as nitric acid,sulfuric acid, acetic acid, or hydrochloric acid. It is also possible toadjust the pH with spent acid process solutions which may or may notcontain additional metals. It is particularly desirable to employ spentacid solutions, such as those derived from the plating process, in thisstage of the waste treatment. This eliminates the need to treat suchsolutions in a separate waste treatment process.

The metal thiocarbamate precipitates usually form a very fine floc whichis not conducive to settling at an appreciable rate and filtration.Thus, it is generally desirable to add both coagulation and flocculationagents to the precipitate for more efficient removal. Suitableflocculation agents useful herein include anionic polymers, such asanionic polyacrylamides. An example of a flocculation agent based on ananionic polyacrylamide is sold under the trademark MetaFloc™ 495 byMorton International, Inc. of Chicago, Ill. Suitable coagulation agentsinclude cationic polymers, such as cationic polyquarternary amines andcationic polyacrylamides. An example of a coagulation agent based oncationic polyquarternary amine is sold under the trademark MetaFloc™ 137by Morton International, Inc. of Chicago, Ill. It is generally desirableto first add from about 50 to 300 ppm of the cationic polymer to thereaction mixture to coagulate the precipitated particulates, followed bythe addition of from about 1 to 10 ppm of the anionic polymer to bridgethe coagulated particulates together to form larger, faster settlingflocs. Usually, it is also desirable to add between about 400 and 2,400ppm of dry diatomaceous earth to the reaction mix. The diatomite acts asa filtering aid by further building-up the size of the particulates.

The treated solution is allowed to settle and the nickel-bearingprecipitate which results from the second stage of the treatment isseparated from the electroless solution by conventional solid-liquidseparation techniques, such as decantation or filtration. Typically, theprecipitated floc is allowed to settle and is then withdrawn from thebottom of the reaction vessel as a sludge. The sludge is thentransported to a filter press to separate the precipitate as a filtercake from the treated liquid. The filter cake containing the insolublenickel thiocarbamate complex can either be mixed with the sludgeproduced from the first step and smelted together, or discardedseparately as hazardous waste in a landfill. Since the amount of sludgeproduced from the second step is minimal, even if it is required todispose of the sludge as hazardous waste due to the presence of metalthiocarbamates, the hazardous waste disposal costs are minimized. Thetwo-step treated liquid can then be pumped directly to a municipalwastewater treatment facility or a natural stream. It is usuallydesirable to filter the treated liquid prior to discharge, sinceresidual particulates may be present in the liquid extracted from thefilter press.

The nickel content in the electroless nickel solution followingthiocarbamate precipitation and prior to discharge is typically aboutca. ≦3 ppm Ni and more typically about ca. ≦0.5 ppm.

In accordance with this invention, the inventors have also surprisinglyfound that incorporation of other metal removal process waste streams,such as zincate waste (a zinc caustic solution) and ion exchangeregenerate waste (a nickel acidic solution), into the spent electrolessnickel feed solutions can be accomplished without adversely affectingthe final nickel levels. The addition of both acidic and basicmetal-bearing streams into the electroless nickel does not alter the pHenough to require additional steps and/or caustic prior to chemicalreduction. This ability to incorporate additional waste streams into theelectroless nickel treatment eliminates the need for separate treatmentsfor each stream and associated costs. Sludge generation is alsosignificantly reduced as a result.

With reference to FIG. 1, the following is a preferred listing ofprocessing steps for waste electroless nickel solutions utilizing theprocess of this invention. It is understood that the exact steps willvary depending on the nature of the waste stream and the equipment beingused in the treatment system. A waste electroless nickel solution 10,along with optional zincate waste solution 12 and optional ion exchangeregenerate waste solution 14, is fed at room temperature to a firstreaction vessel 16 equipped with a stirrer 18. Under agitation, sodiummetabisulfite 20 and lime 22 to facilitate reduction and filtration areadded and the pH of the solution is then raised to the 7-11 range withthe addition of sodium hydroxide 24. Still under agitation, excessstabilized sodium borohydride reduction solution 26 (VenMet® solution)is added. Agitation is continued until substantial completion of theborohydride reduction reaction. The borohydride reduction reaction canbe represented by the following equation:

    8Ni.sup.+2 X+NaBH.sub.4 +2H.sub.2 O→NaBO.sub.2+8 HX+8Ni.sup.0.sub.(s)

where X=the anion (chloride, sulfate, carbonate, acetate). Aftercompletion, the agitation is discontinued and the solid precipitate isallowed to settle as a slurry or sludge 28. The sludge 28 can be drawnoff the bottom of the reaction vessel 16 and fed to a filter press 30where the treated liquid is separated from the solid precipitate. Thefilter cake 32 which contains high concentrations of solid nickelcompounds is drummed and set aside for subsequent nickel recovery by asmelting operation (not shown).

Next, the treated liquid 34 containing substantially reduced levels ofnickel and being essentially free of solids is then fed at roomtemperature to a second reaction vessel 36 equipped with a stirrer 38.Under agitation, nitric acid 40, which may be drawn from a spent acidstream, is added to lower the pH of the solution to within the range of5-8. Still under agitation, excess sodium dimethyldithiocarbamate 42(Metal Plex™ 143) is added. Agitation is continued until substantialcompletion of the organosulfur precipitation reaction.

The organosulfur precipitation reaction is represented by the followingequation:

    2DTC-Na+Ni.sup.+2 (complexed)→2DTC-Ni.sub.(s).

After completion, still under agitation a coagulant 44 (MetaFloc™ 137)is added and mixed until sufficient coagulation, then a flocculant 46(MetaFloc™ 495) is added and mixed until sufficient flocculation, andlastly sufficient dry diatomaceous earth 48 is added and mixed until thedesired build-up of the precipitate is complete. The agitation is thendiscontinued and the solid organosulfur precipitate is allowed to settleas a slurry or sludge 50. The sludge 50 can then be drawn off the bottomof the reaction vessel and fed to a filter press 52 where the treatedliquid is separated from the solid precipitate. The filter cake 54containing essentially all remaining nickel is minimal and can becombined with the filter cake 32 that resulted from the borohydridetreatment. The treated liquid 56 is then fed through a filter 58 (0.2micron filter) to ensure removal of any residual solid particulates fromthe solution. Also, as shown in FIG. 1, at start-up the treated liquid56 may be returned back to the second reaction vessel 36 instead ofbeing initially passed through the filter 58 to prevent unwantedclogging of the filter. The filtered liquid 60 can then be safelydischarged directly into a wastewater treatment plant stream or anatural stream (not shown).

The following non-limiting examples will now illustrate theeffectiveness of the process of this invention in renderingmetal-bearing solutions safe for discharge.

EXAMPLE 1 Waste Treatment of Spent Electroless Nickel Solution

500 mL of spent electroless nickel solution was placed in a beaker witha magnetic stir bug. 250 ppm Na₂ S₂ O₅ was added to the electrolessnickel solution. The pH of the solution was then raised from pH 5 to10.5 with 500 ppm of lime and 50% NaOH solution. 6 mL of VenMet®reduction solution (12 mL/L) was placed in a small beaker and dilutedapproximately 2:1 with water. The diluted VenMet® solution was poureddirectly into the pH adjusted electroless nickel solution. The smallbeaker was rinsed once with water and the rinse was also poured into theelectroless nickel solution. Using moderate agitation the solution wasmixed for 4 hours. After the 4 hours the electroless nickel solution wasremoved from agitation and filtered through a #4 Whatman® filter paperto remove the solids. The pH of the solution was then lowered to pH 7with 35% nitric acid solution. 4 mL of Metal Plex™ 143 (20 mL/L) wasadded to the reduced and filtered solution. This was allowed to mix for1 hour using moderate agitation. After the 1 hour 100 ppm of MetaFloc™137 was added to the solution and allowed to mix for 10 minutes. Then 2ppm of MetaFloc™ 495 was added and allowed to mix for 10 minutes.Lastly, 2 mL (dry) diatomaceous earth (10 mL/L) was added and allowed tomix for 10 minutes. The treated solution was then filtered through a 0.2μm syringe filter for analysis. The final nickel content in the treatedsolution was ca. <0.1 ppm.

EXAMPLE 2 Waste Treatment of Combined Spent Electroless Nickel andZincate Waste Solutions

450 mL of spent electroless nickel solution was placed in a beaker witha magnetic stir bug. 50 mL of waste zincate solution was also placedinto the beaker. The pH of the solution was then raised from pH 5.6 to10.5 with 50% NaOH solution. 6 mL of VenMet® reduction solution (12mL/L)was placed in a small beaker and diluted approximately 2:1 with water.The diluted VenMet® solution was poured directly into the pH adjustedelectroless nickel solution. The small beaker was rinsed once with waterand the rinse was also poured into the electroless nickel solution.Using moderate agitation the solution was mixed for 4 hours. After the 4hours the solution was removed from agitation and filtered through a #4Whatman® filter paper to remove the solids. The pH of the solution wasthen lowered to pH 8 with 35% nitric acid solution. 10 mL of Metal Plex™143 (50 mL/L) was added to the reduced and filtered solution and allowedto mix for 1 hour using moderate agitation. After the 1 hour 100 ppm ofMetaFloc™ 137 was added to the solution and mixed for 10 minutes. Then 2ppm of MetaFloc™ 495 was added and mixed for 10 minutes. Lastly, 2 mL(dry) diatomaceous earth (10 mL/L) was added and mixed for 10 minutes.The treated solution was then filtered through a 0.2 μm syringe filterfor analysis. The final nickel content in the treated solution was ca.<0.1 ppm and the final zinc content in solution was ca. 0.16 ppm.

The overall efficiency, simplicity, economic and ecological gains, andlegal benefits which contribute to making the process of this inventionmost desirable for removing dissolved heavy metals from metal-bearingsolutions should now be apparent to persons skilled in the art.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are apparent and inherent. Since manypossible variations may be made of the invention without departing fromthe scope thereof, the invention is not intended to be limited to theembodiments and examples disclosed, which are considered to be purelyexemplary. Accordingly, reference should be made to the appended claimsto assess the true spirit and scope of the invention, in which exclusiverights are claimed.

What is claimed is:
 1. A process for the waste treatment of ametal-bearing waste solution to reduce the dissolved metal content priorto discharge, comprising the sequential steps of contacting the solutionwith a reducing agent for a sufficient time to precipitate a substantialportion of said dissolved metal content by chemical reduction, followedby contacting said solution with a precipitating agent for a sufficienttime to precipitate substantially all of the remaining dissolved heavymetal content by organosulfur precipitation, then separating theprecipitated metal compounds from the treated solution, and finallydischarging the treated solution into a waste stream, wherein prior toorganosulfur precipitation an intermediate separation is performed toremove substantially all of the reduction precipitate from saidsolution.
 2. The process of claim 1, wherein said reducing agentcomprises sodium borohydride.
 3. The process of claim 1, wherein saidprecipitating agent comprises sodium dimethyldithiocarbamate.
 4. Theprocess of claim 1, wherein said metal-bearing waste solution comprisesone or more solutions including at least a spent electroless nickelplating solution.
 5. A process for the waste treatment of a spent metalplating solution to reduce the dissolved metal content to dischargeablelevels, comprising sequentially contacting said solution with aborohydride reducing agent for a sufficient time to precipitate asubstantial portion of the dissolved metal content in said solution,intermediately separating the reduced metal precipitate from saidsolution, followed by contacting said solution with a thiocarbamateprecipitating agent for a sufficient time to precipitate substantiallyall of the remaining dissolved metal content in said solution,separating the precipitated metal thiocarbamates from said solution, andfinally discharging said solution directly into a waste stream.
 6. Theprocess of claim 5, wherein said borohydride reducing agent comprisessodium borohydride.
 7. The process of claim 5, wherein saidthiocarbamate precipitating agent comprises sodiumdimethyldithiocarbamate.
 8. The process of claim 5, wherein said processis run in a batchwise manner.
 9. The process of claim 5, wherein saidprocess is run in a continuous manner.
 10. The process of claim 5,wherein said metal plating solution comprises a spent electroless nickelplating solution.
 11. The process of claim 5, wherein the metalcontained in said reduced metal precipitate is recoverable.
 12. Theprocess of claim 5, wherein the pH of said solution is adjusted tobetween about 4 and about 11 prior to borohydride reduction.
 13. Theprocess of claim 5, wherein the pH of said solution is adjusted tobetween about 5 and about 8 prior to thiocarbamate precipitation. 14.The process of claim 5, wherein said metal thiocarbamate precipitatesare contacted with a filtering aid selected from the group consisting ofcoagulants, flocculants, and diatomaceous earth, to facilitatesubsequent separation.
 15. The process of claim 5, wherein prior toborohydride reduction, sodium metabisulfite and lime are added to saidsolution to facilitate reduction and intermediate separation.
 16. Theprocess of claim 5, wherein said borohydride reducing agent and saidthiocarbamate precipitating agent are each provided in excess ofstoichiometric amount based on the dissolved metal content of saidsolution.
 17. The process of claim 10, wherein the discharged solutioncontains less than 3 ppm dissolved nickel content.
 18. The process ofclaim 10, wherein the discharged solution contains less than 0.5 ppmdissolved nickel content.
 19. A process for the waste treatment of aspent electroless nickel plating solution to reduce the dissolved nickelcontent to dischargeable levels, comprising adjusting the pH of saidsolution to between about 7 and 11, contacting said solution with excesssodium borohydride reducing agent for a sufficient time to precipitate asubstantial portion of the dissolved nickel content in said solution,intermediately separating the reduced nickel precipitate from saidsolution, pH adjusting said solution to between about 5 and 8, followedby contacting said solution with excess sodium dimethyldithiocarbamateprecipitating agent for a sufficient time to precipitate substantiallyall of the remaining dissolved nickel content in said solution,contacting said solution with a filtering aid selected from the groupconsisting of coagulants, flocculants, and diatomaceous earth,separating the precipitated metal thiocarbamates from said solution, andfinally discharging said solution directly into a waste stream.
 20. Theprocess of claim 19, wherein prior to borohydride reduction, an oxidizerscavenging agent and lime are added to said solution to facilitatereduction and intermediate separation.